it says that y = mx + b, so y is alone, to make y alone, move 6x to the other side by subtracting it from both sides
-5y = 15 - 6x
its almost y = mx + b, except for the fact that y has a coefficient, so let's try dividing by it?
-5/-5 y = 15/-5 - 6/-5 x
y = -3 + 6x/5
or y = 6/5 x -3